Office for Mac

I have Office for Mac (2015) (from Charity Digital) on my MacBook pro (2013)     I have had to replace it with a MacBook pro (2020).  I have migrated everything but Office says it will not work and needs upgrading

I am an unpaid trustee & director of 3 charities and I don't want to go onto 365 - how can I get my current version working or updated?